Optical fiber connectors are used to route light from one optical fiber to another, or to connect an optical fiber to a device such as a light source or a detector. When optically connecting one optical fiber to another, the cores of the two optical fibers need to be accurately aligned with one another so that the maximum amount of light is transferred between the fiber cores, i.e., so that the insertion loss is minimized. For single-mode fibers at telecommunications wavelengths (e.g. 1310 nm, 1550 nm), the core diameter is about 8 microns and the mode-field diameter is about 10 microns. To keep the insertion loss below 0.25 dB (˜5%), the radial offset between the two coupled fibers needs to be less than 1.1 microns.
To facilitate meeting this alignment tolerance, optical fiber connectors include a ferrule that holds the optical fiber. A ferrule is a cylindrical sleeve having a central bore sized to accommodate the bare (stripped) end of the optical fiber. The ferrule typically has a diameter ranging from 1.25 mm to 2.50 mm, and is precision made so that the central bore is located to within about 1 micron of true center, and the ferrule outer surface is within about 0.5 micron of true round. The ferrule is generally made of a hard material, e.g., a ceramic such as zirconia, so that it can be mechanically held and serve as a mechanical reference when engaging the connector with another connector or with a device.
There are a number of different sources of insertion loss, including fiber bends, poor polishing, contamination of the connectors' surfaces, fiber endface angles and shapes, and tolerance errors on the fiber core position relative to the ferrule true center, which is referred to herein as the core-ferrule concentricity. It is therefore important to have a fast and accurate way of measuring the position of the fiber core within the ferrule so that this particular loss mechanism can be identified and characterized to see if it is a limiting factor in establishing a suitable optical connection. The measurement of core-ferrule concentricity has become particularly important in view of increasingly stringent requirements on connector insertion loss.